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  异常处理
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  <h1 id="异常"><a href="#异常" class="headerlink" title="异常"></a>异常</h1><hr>
<h2 id="异常类型"><a href="#异常类型" class="headerlink" title="异常类型"></a>异常类型</h2><h3 id="Throwable-类"><a href="#Throwable-类" class="headerlink" title="Throwable 类"></a>Throwable 类</h3><p>Java 程序中的异常是一个在程序执行期间发生的事件，它中断正在执行程序的正常指令流。为了能够及时有效地处理程序中的运行错误，必须使用异常类。</p>
<p>java 程序中所有的异常都继承自 Throwable 类，Throwable 类有两个子类 Error 类和 Exception 类：</p>
<ul>
<li><p><strong>Error 类</strong>：【错误】表示 java 程序在运行时产生的无法处理的故障（如堆栈溢出），错误出现时会导致程序无法正常执行并强制退出。</p>
</li>
<li><p><strong>Exception 类</strong>：【异常】表示 java 程序中产生的可以被处理的故障，异常出现时可以由程序进行处理。</p>
</li>
</ul>
<p><img src="/pic/exception.png" alt="Throwable"></p>
<h3 id="RuntimeException-类"><a href="#RuntimeException-类" class="headerlink" title="RuntimeException 类"></a>RuntimeException 类</h3><p>【运行时异常】 Exception 类的子类。</p>
<p>表示 java 程序运行状态中发生的异常，在编译时无法被检测。在 java 程序运行时会由系统自动抛出，允许应用程序不进行处理。</p>
<table>
<thead>
<tr>
<th>异常类型</th>
<th>介绍</th>
</tr>
</thead>
<tbody><tr>
<td>ArithmeticException</td>
<td>算术异常，以零做除数</td>
</tr>
<tr>
<td>ArrayIndexOutOfBoundException</td>
<td>数组越界异常</td>
</tr>
<tr>
<td>NullPointerException</td>
<td>空指针异常，对象不存在</td>
</tr>
</tbody></table>
<h3 id="Checked-Exception-类"><a href="#Checked-Exception-类" class="headerlink" title="Checked Exception 类"></a>Checked Exception 类</h3><p>【可检查异常】Exception 类除 RuntimeException 以外其他子类的统称。</p>
<p>表示 java 程序编译时检测到的异常。出现时必须在程序中进行捕获或抛出，否则编译不会通过。</p>
<table>
<thead>
<tr>
<th>异常类型</th>
<th>介绍</th>
</tr>
</thead>
<tbody><tr>
<td>IOException</td>
<td>IO 异常</td>
</tr>
<tr>
<td>FileNotFoundException</td>
<td>找不到文件异常，继承自 IO 异常</td>
</tr>
<tr>
<td>ClassNotFoundException</td>
<td>找不到类异常</td>
</tr>
</tbody></table>
<hr>
<h2 id="Exception-类"><a href="#Exception-类" class="headerlink" title="Exception 类"></a>Exception 类</h2><h3 id="源码解析"><a href="#源码解析" class="headerlink" title="源码解析"></a>源码解析</h3><p><strong>状态信息</strong></p>
<p>Throwable / Exception 类是有状态的（因此 Throwable 是接口而不能是类），记录了四个信息：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">private</span> <span class="keyword">transient</span> Object backtrace;                          <span class="comment">// 栈的回溯点</span></span><br><span class="line"><span class="keyword">private</span> String detailMessage;                                <span class="comment">// 异常的信息：在创建异常时备注</span></span><br><span class="line"><span class="keyword">private</span> <span class="type">Throwable</span> <span class="variable">cause</span> <span class="operator">=</span> <span class="built_in">this</span>;                              <span class="comment">// 异常的原因：导致该异常的异常，默认为自身</span></span><br><span class="line"><span class="keyword">private</span> StackTraceElement[] stackTrace = UNASSIGNED_STACK;   <span class="comment">// 异常的发生顺序：以栈的形式存储</span></span><br></pre></td></tr></table></figure>

<p><strong>构造方法</strong></p>
<p>Throwable / Exception 类含有四个构造方法，在创建时可以记录异常信息：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">throw</span> <span class="keyword">new</span> <span class="title class_">Exception</span>();                           <span class="comment">// 默认</span></span><br><span class="line"><span class="keyword">throw</span> <span class="keyword">new</span> <span class="title class_">Exception</span>(<span class="string">&quot;message&quot;</span>);                  <span class="comment">// 记录异常信息</span></span><br><span class="line"><span class="keyword">throw</span> <span class="keyword">new</span> <span class="title class_">Exception</span>(e);                          <span class="comment">// 记录异常原因</span></span><br><span class="line"><span class="keyword">throw</span> <span class="keyword">new</span> <span class="title class_">Exception</span>(<span class="string">&quot;message&quot;</span>, e);               <span class="comment">// 记录详细信息和异常原因</span></span><br></pre></td></tr></table></figure>

<p><strong>常用方法</strong></p>
<p>Throwable / Exception 类定义了多种常用方法用于获取异常数据，常用的有：</p>
<ul>
<li>getMessage 方法：获取异常的信息。</li>
<li>getStackTrace 方法：获取的异常发生顺序。</li>
<li>printStackTrace 方法：获取异常的发生顺序并打印（开发和调试阶段用来显示异常信息，帮助开发者找出错误）。</li>
</ul>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">catch</span>(Exception e)&#123;</span><br><span class="line">    System.out.println(e.getMessage());</span><br><span class="line">    e.printStacTrace();                           </span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<h3 id="自定义异常"><a href="#自定义异常" class="headerlink" title="自定义异常"></a>自定义异常</h3><p>我们也可以通过继承并重写 Exception / RuntimeException 类的方式，自定义异常类并使用。</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// 自定义异常，重写方法可任选</span></span><br><span class="line"><span class="keyword">class</span> <span class="title class_">MyException</span> <span class="keyword">extends</span> <span class="title class_">Exception</span> &#123;</span><br><span class="line">    <span class="meta">@Override</span></span><br><span class="line">    <span class="keyword">public</span> <span class="title function_">MyException</span><span class="params">()</span> &#123;</span><br><span class="line">        <span class="built_in">super</span>();</span><br><span class="line">    &#125;</span><br><span class="line">    <span class="meta">@Override</span></span><br><span class="line">    <span class="keyword">public</span> <span class="title function_">MyException</span><span class="params">(String message)</span> &#123;</span><br><span class="line">        <span class="built_in">super</span>(message);</span><br><span class="line">    &#125; </span><br><span class="line">    <span class="meta">@Override</span></span><br><span class="line">    <span class="keyword">public</span> <span class="title function_">MyException</span><span class="params">(String message, Throwable cause)</span>&#123;</span><br><span class="line">        <span class="built_in">super</span>(message,cause);</span><br><span class="line">    &#125;</span><br><span class="line">    <span class="meta">@Override</span></span><br><span class="line">    <span class="keyword">public</span> <span class="title function_">MyException</span><span class="params">(Throwable cause)</span> &#123;</span><br><span class="line">        <span class="built_in">super</span>(cause);</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<h3 id="异常转译"><a href="#异常转译" class="headerlink" title="异常转译"></a>异常转译</h3><p>在项目开发过程中，当 Sevice/DAO 层出现如 SQLException 异常时，程序一般不会把底层的异常传到 controller 层。程序可以捕获原始异常，然后再抛出一个新的业务异常。</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">catch</span>(SQLException e)&#123;</span><br><span class="line">    <span class="keyword">throw</span> <span class="keyword">new</span> <span class="title class_">MyException</span>(<span class="string">&quot;SQL Error&quot;</span>, e);</span><br><span class="line">&#125;</span><br><span class="line"></span><br></pre></td></tr></table></figure>


<hr>
<h2 id="异常处理"><a href="#异常处理" class="headerlink" title="异常处理"></a>异常处理</h2><h3 id="抛出异常-throw"><a href="#抛出异常-throw" class="headerlink" title="抛出异常 throw"></a>抛出异常 throw</h3><p>当方法执行出现问题时，方法就会创建异常对象并抛出。开发者可以在程序中自行抛出异常；JVM 在执行程序时发现问题也会自动抛出异常。</p>
<ul>
<li><p><strong>throw 语句</strong>：开发者自行创建异常对象并抛出，等待程序进行异常处理。</p>
</li>
<li><p><strong>throws 语句</strong>：声明方法可能抛出某种异常且未经处理，调用该方法的上级需要进行异常处理。</p>
</li>
</ul>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br></pre></td><td class="code"><pre><span class="line"></span><br><span class="line"><span class="keyword">class</span> <span class="title class_">TestException</span>&#123;       </span><br><span class="line">    <span class="comment">// 把方法中的抛出异常交给上层处理     </span></span><br><span class="line">    <span class="keyword">public</span> <span class="keyword">void</span> <span class="title function_">writeList</span><span class="params">(<span class="type">int</span> size)</span> <span class="keyword">throws</span> IndexOutOfBoundsException, IOException&#123;</span><br><span class="line">        <span class="type">PrintWriter</span> <span class="variable">out</span> <span class="operator">=</span> <span class="literal">null</span>;</span><br><span class="line">        <span class="comment">// 用户自定义异常并抛出</span></span><br><span class="line">        <span class="keyword">if</span>(size &lt; <span class="number">1</span>) <span class="keyword">throw</span> <span class="keyword">new</span> <span class="title class_">IndexOutOfBoundsException</span>(<span class="string">&quot;至少要输出1个字符&quot;</span>);</span><br><span class="line">        <span class="keyword">try</span>&#123;</span><br><span class="line">            <span class="comment">// 虚拟机自动发现异常也会抛出，必须出现在 try 代码块中</span></span><br><span class="line">            out = <span class="keyword">new</span> <span class="title class_">PrintWriter</span>(<span class="keyword">new</span> <span class="title class_">FileWriter</span>(txt));</span><br><span class="line">            <span class="keyword">for</span> (<span class="type">int</span> <span class="variable">i</span> <span class="operator">=</span> <span class="number">0</span>; i &lt; size; i++)</span><br><span class="line">                System.out.println(<span class="string">&quot;Value at: &quot;</span> + i + <span class="string">&quot; = &quot;</span> + list.get(i));</span><br><span class="line">        &#125;<span class="keyword">finally</span>&#123;</span><br><span class="line">            <span class="keyword">if</span> (out != <span class="literal">null</span>) out.close();</span><br><span class="line">        &#125;</span><br><span class="line">     &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<h3 id="捕获异常-catch"><a href="#捕获异常-catch" class="headerlink" title="捕获异常 catch"></a>捕获异常 catch</h3><p>当方法执行抛出异常时，必须由专门的代码块对异常进行处理。</p>
<ul>
<li><p><strong>try 语句</strong>：可能出现异常的代码块。</p>
</li>
<li><p><strong>catch 语句</strong>：捕获相应异常后停止执行 try 代码，转而执行对应 catch 代码。如果没有异常 catch 代码不会执行。</p>
</li>
<li><p><strong>finally 语句</strong>：无论是否发生异常，finally 代码总会被执行。一般用于释放资源。</p>
</li>
</ul>
<p><strong>注意事项</strong></p>
<ol>
<li><p>如果 try 语句中出现的异常未被 catch，默认将异常 throw 给上层调用者处理。但必须在方法中声明 throws。</p>
</li>
<li><p>try/catch 代码中的 return 语句会在执行完 finally 后再返回，但 finally 中对返回变量的改变不会影响最终的返回结果。</p>
</li>
<li><p>finally 代码中应避免含有 return 语句或抛出异常，否则只会执行 finally 中的 return 语句，且不会向上级抛出异常。</p>
</li>
</ol>
<p><em>Java 7 后在 try 语句中打开 IO 流，会在跳出后自动关闭流。不必再用 finally 语句关闭。</em></p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">class</span> <span class="title class_">TestException</span>&#123;               </span><br><span class="line">    <span class="keyword">public</span> <span class="keyword">void</span> <span class="title function_">writeList</span><span class="params">(<span class="type">int</span> size)</span> &#123;</span><br><span class="line">        <span class="type">PrintWriter</span> <span class="variable">out</span> <span class="operator">=</span> <span class="literal">null</span>;</span><br><span class="line">        <span class="keyword">try</span> &#123;</span><br><span class="line">            <span class="keyword">if</span>(size &lt; <span class="number">1</span>) <span class="keyword">throw</span> <span class="keyword">new</span> <span class="title class_">IndexOutOfBoundsException</span>(<span class="string">&quot;至少要输出1个字符&quot;</span>);</span><br><span class="line">            out = <span class="keyword">new</span> <span class="title class_">PrintWriter</span>(<span class="keyword">new</span> <span class="title class_">FileWriter</span>(<span class="string">&quot;OutFile.txt&quot;</span>));</span><br><span class="line">            <span class="keyword">for</span> (<span class="type">int</span> <span class="variable">i</span> <span class="operator">=</span> <span class="number">0</span>; i &lt; size; i++)</span><br><span class="line">                System.out.println(<span class="string">&quot;Value at: &quot;</span> + i + <span class="string">&quot; = &quot;</span> + list.get(i));</span><br><span class="line">        &#125; <span class="keyword">catch</span> (IndexOutOfBoundsException e) &#123;</span><br><span class="line">            System.err.println(<span class="string">&quot;Caught IndexOutOfBoundsException: &quot;</span> + e.getMessage());</span><br><span class="line">        &#125; <span class="keyword">catch</span> (IOException e) &#123;</span><br><span class="line">            System.err.println(<span class="string">&quot;Caught IOException: &quot;</span> + e.getMessage());</span><br><span class="line">        &#125; <span class="keyword">finally</span> &#123;</span><br><span class="line">            <span class="keyword">if</span> (out != <span class="literal">null</span>) out.close();</span><br><span class="line">        &#125;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<hr>
<!--

**异常处理类**

在 Spring Boot 中，所有异常统一由专门的异常处理类`@ControllerAdvice`处理。

<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">//控制器异常处理类</span></span><br><span class="line"><span class="meta">@ControllerAdvice</span> </span><br><span class="line"><span class="keyword">public</span> <span class="keyword">class</span> <span class="title class_">ErrorHandler</span> &#123;</span><br><span class="line">    </span><br><span class="line">    <span class="keyword">private</span> <span class="keyword">static</span> <span class="keyword">final</span> org.slf4j.<span class="type">Logger</span> <span class="variable">log</span> <span class="operator">=</span> LoggerFactory.getLogger(ErrorHandler.class);</span><br><span class="line">    </span><br><span class="line">    <span class="comment">//输入参数校验异常</span></span><br><span class="line">    <span class="meta">@ExceptionHandler(value = MethodArgumentNotValidException.class)</span></span><br><span class="line">    <span class="keyword">public</span> ResponseEntity&lt;Result&gt; <span class="title function_">NotValidExceptionHandler</span><span class="params">(HttpServletRequest req, MethodArgumentNotValidException e)</span> <span class="keyword">throws</span> Exception &#123;</span><br><span class="line">        </span><br><span class="line">        log.debug(<span class="string">&quot;异常详情&quot;</span>, e);</span><br><span class="line">        <span class="type">BindingResult</span> <span class="variable">bindingResult</span> <span class="operator">=</span> e.getBindingResult();</span><br><span class="line">        </span><br><span class="line">        <span class="comment">//rfc4918 - 11.2. 422: Unprocessable Entity          </span></span><br><span class="line">        <span class="type">Result</span> <span class="variable">res</span> <span class="operator">=</span> MiscUtil.getValidateError(bindingResult);</span><br><span class="line">        <span class="keyword">return</span> <span class="keyword">new</span> <span class="title class_">ResponseEntity</span>&lt;Result&gt;(res, HttpStatus.UNPROCESSABLE_ENTITY);</span><br><span class="line">    &#125;</span><br><span class="line">    </span><br><span class="line">    <span class="comment">//404异常处理</span></span><br><span class="line">    <span class="meta">@ExceptionHandler(value = NoHandlerFoundException.class)</span></span><br><span class="line">    <span class="keyword">public</span> ResponseEntity&lt;Result&gt; <span class="title function_">NoHandlerFoundExceptionHandler</span><span class="params">(HttpServletRequest req, Exception e)</span> <span class="keyword">throws</span> Exception &#123;</span><br><span class="line">        </span><br><span class="line">        log.debug(<span class="string">&quot;异常详情&quot;</span>, e);</span><br><span class="line">                </span><br><span class="line">        <span class="type">Result</span> <span class="variable">res</span> <span class="operator">=</span> <span class="keyword">new</span> <span class="title class_">Result</span>(<span class="number">404</span>, <span class="string">&quot;页面不存在&quot;</span>);</span><br><span class="line">        <span class="keyword">return</span> <span class="keyword">new</span> <span class="title class_">ResponseEntity</span>&lt;Result&gt;(res, HttpStatus.NOT_FOUND);</span><br><span class="line">    &#125;</span><br><span class="line">    </span><br><span class="line">    <span class="comment">//其他默认异常处理</span></span><br><span class="line">    <span class="meta">@ExceptionHandler(value = Throwable.class)</span></span><br><span class="line">    <span class="keyword">public</span> ResponseEntity&lt;Result&gt; <span class="title function_">defaultHandler</span><span class="params">(HttpServletRequest req, Exception e)</span> <span class="keyword">throws</span> Exception &#123;</span><br><span class="line">                </span><br><span class="line">        <span class="type">Result</span> <span class="variable">res</span> <span class="operator">=</span> <span class="keyword">new</span> <span class="title class_">Result</span>(<span class="number">500</span>, <span class="string">&quot;服务器内部错误&quot;</span>);</span><br><span class="line">        log.debug(<span class="string">&quot;异常详情&quot;</span>, e);</span><br><span class="line">        </span><br><span class="line">        <span class="keyword">return</span> <span class="keyword">new</span> <span class="title class_">ResponseEntity</span>&lt;Result&gt;(res, HttpStatus.INTERNAL_SERVER_ERROR);</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>–&gt;</p>
 
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